KVM: x86: Suppress MMIO that is triggered during task switch emulation

Explicitly suppress userspace emulated MMIO exits that are triggered when
emulating a task switch as KVM doesn't support userspace MMIO during
complex (multi-step) emulation.  Silently ignoring the exit request can
result in the WARN_ON_ONCE(vcpu->mmio_needed) firing if KVM exits to
userspace for some other reason prior to purging mmio_needed.

See commit 0dc902267cb3 ("KVM: x86: Suppress pending MMIO write exits if
emulator detects exception") for more details on KVM's limitations with
respect to emulated MMIO during complex emulator flows.

Reported-by: syzbot+2fb9f8ed752c01bc9a3f@syzkaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240712144841.1230591-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index caf528bb775b09bfe410b816ebdac6de84726bc2..6c07f7ff0eff623726c8d2e168c8590edc46e95c 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11800,7 +11800,13 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
 
        ret = emulator_task_switch(ctxt, tss_selector, idt_index, reason,
                                   has_error_code, error_code);
-       if (ret) {
+
+       /*
+        * Report an error userspace if MMIO is needed, as KVM doesn't support
+        * MMIO during a task switch (or any other complex operation).
+        */
+       if (ret || vcpu->mmio_needed) {
+               vcpu->mmio_needed = false;
                vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
                vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
                vcpu->run->internal.ndata = 0;